This isn't a rule about being required to reply. It's a rule about not offering new arguments until old arguments have been accepted or refuted.

I only meant the rule to apply to interactions-- A offers argument A1, B (who's discussing the matter with A) must address A! before moving on to B1. C (who hasn't said anything so far) is under no obligation.

If B didn't see A1 or doesn't remember it, then B should be politely reminded of it. If B then persists in offering B1, then the rule gets invoked.